Hi Karl,

I think the design you have posted with rails their side would still work OK, especially for hobby use. But to elaborate a bit further . . .

Considering the forces - there are 2 forces acting on the bearings, one from the mass of the bits they are carrying (router, motors etc.), and one from the cutting forces. I've ignored the forces due to accelerations because some early calcs showed these to be small, especially on a hobby machine.

The mass of the bits is huge compared to the cutting forces (especially wood) so I would orient the bearings to take this larger load. In this case I would favour Jonathan's arrangement, top and bottom.

In comparing 4 bearings on their side with 2 bearings on top (and 2 guide bearings on bottom), I would agree that the top/bottom arrangement puts more load on the top 2 bearings vs spreading it over the 4 side bearings, but I wouldn't worry about exceeding the load rating because you are probably still miles away. Instead consider what happens to that load when it is applied to the stiffness of the bearing in that direction. Arranged on their side the large mass of the Z axis is trying to open up the bearings, thereby slightly reducing their preload or grip on the rail. This would be like having very slightly smaller ball bearings inside the unit which would slightly reduce the accuracy of the carraige on the rail. Arranged on top and bottom you are mainly loading the bearing in the stiffest direction, so would expect that to give the best overall results even though it is carrying more load per bearing.